Fill gap after circular cut
The easiest way is to use the 'Bridge Edge Loops' tool in the specials menu, accessed by W.


Select the edges around your holes on both sides of the mesh (click to select an edge whilst holding Alt to select edge loops).
Open the specials menu W.
Select "Bridge Edge Loops" and you should end up with the result you desire.




Alternatively, you could simply use a boolean modifier with a cylinder. This will automatically create the internal faces.